During the course of our investigation of the mesenchymal-epithelial transition induced by enforced expression of the WT1 tumor suppressor gene, a gene indispensable for renal development, we isolated the murine sprouty1 gene as a WT1 target gene. The Drosophila sprouty (spry) gene is an inhibitor of branching morphogenesis in the Drosophila tracheal system. Loss of spry leads to increased branching and a loss of signal inhibition while loss of FGF in the fly leads to no branching. Branching morphogenesis also occurs as the ureteric bud invades the metanephric mesenchyme in the developing vertebrate kidney. We found spry1 to be expressed in the developing ureteric bud as well as the comma and S-shaped body of the murine kidney. We found that sprouty proteins potently inhibit the activation of ras in response to growth factors and receptor tyrosine kinases while leaving the phosphatidyl inositol/Akt pathway intact. Preliminary data also indicate that sprouty can interfere with activation of MAP kinase through integrin stimulation. We deleted the sprouty1 gene in mice and the result was a striking phenotype of polycystic kidneys. In this proposal we will characterize the nature of these cysts in terms of their cell of origin, time course of development and the nature of the proliferative and/or apoptotic defect in the renal cystic epithelium. We have found sprouty1 in immunoprecipitates of Pkd1 and hence we believe that this protein may play a role in the signals transduced by the Pkd1/2 complex. To determine if the PKD complex is in the same genetic pathway as sprouty1 we will cross Pkd2 and sprouty1 mutant mice and determine the resulting phenotype. Lastly we will perform a proof of principal experiment of gene therapy of PKD with sprouty1. We will replace sprouty1 in sprouty1-null embryonic kidneys in culture and determine if this can reverse the formation of cysts. We will then determine if sprouty1 can affect cyst formation in Pkd2 null kidneys. Together these studies will increase our knowledge of the signaling pathways involved in renal cyst formation and could validate sprouty1 as a novel therapeutic agent/target for polycystic kidney disease.